(1) Field of the Invention
The present invention relates to an optical output control method for use in an optical transmission node and an optical output control apparatus for use in the same. In more particularly, the present invention relates to a technology suitable for use in an optical transmission node with an automatic optical output control (APSD: Automatic Power Shut-Down) function which controls, in response to a reception of optical path cut (light down) detecting information from another optical transmission node, the optical output therefrom to decrease or halt.
(2) Description of Related Art
FIG. 10 is a block diagram showing one example of a conventional WDM (Wavelength Division Multiplex) optical transmission system. As shown in FIG. 10, the WDM optical transmission system comprises an optical transmission station (optical transmission node) 100A (station A) and an optical transmission station (optical transmission node) 100B (station B) connected to each other through a pair of (couple of) optical transmission paths (fiber cables) so that bidirectional communication can be executed. In the system shown in FIG. 10, the optical transmission node 100B (station B) is also connected to another neighboring optical transmission node (optical node or a terminal equipment: station C) so that bidirectional communication can be executed between the stations.
In order to attain the bidirectional communication between the optical transmission nodes (hereinafter referred to as simply “node” or “station”) 100A and 100B, the nodes have a symmetrical arrangement. That is, each of the nodes has both of a transmission system and a reception system. The transmission system comprises WDM couplers 101S and 103S, an optical amplifier 102S, a receiving module 104S and transmission module 105S for monitoring control, and the reception system comprises WDM couplers 101R and 103R, an optical amplifier 102R, a receiving module 104R and transmission module 105R for monitoring control.
In the transmission system, the WDM coupler 101S is a unit for extracting a monitoring control signal as an OSC (Optical Service Channel) contained in a received WDM signal light and supplying the monitoring control signal to the receiving module 104S. The optical amplifier 102S is a unit for amplifying the optical output power to a necessary level which enables the received WDM signal light supplied from the WDM coupler 101S to be sent to the node B (or 100A) in the subsequent stage depending on the transmission distance between the source to the destination. This amplification is performed in a manner that the wavelengths are collectively set to the same wavelength. The WDM coupler 103S is a unit for coupling the OSC light (monitoring control signal) from the transmission module 105S and the WDM signal light after undergoing amplification in the optical amplifier 102S to each other and outputting the resultant signal to the optical transmission path 200 (or 300).
The receiving module 104S is a unit for receiving an OSC light transmitted from the WDM coupler 101S and performing control in accordance with the monitoring control signal (APSD control or the like which will be described later on). The receiving module 105S is a unit for generating information to be notified for the next node (station located on the upstream side) 100B (or 100A) as the monitoring control signal.
Conversely, in the receiving system, the WDM coupler 101R is a unit for branching the OSC signal (monitoring control signal) contained in the received WDM signal supplied from the optical transmission path 300 or 200 to the receiving module 104R. The optical amplifier 102R is a unit for amplifying the optical output power to a necessary level which enables the received WDM signal light supplied from the WDM coupler 101R to reach the node in the subsequent stage depending on the transmission distance between the source to the destination. This amplification is performed in such a manner that the wavelengths are collectively set to the same wavelength. The WDM coupler 103R is a unit for coupling the OSC light (monitoring control signal) from the transmission nodule 105R and the WDM signal light after undergoing amplification in the optical amplifier 102R to each other.
The receiving module 104R is a unit for receiving the OSC light (monitoring control signal) transmitted from the WDM coupler 101R and performing control in accordance with the monitoring control signal (APSD control or the like). The transmitting module 105R is a unit for generating information to be notified for the next node (station located on the upstream side) as the monitoring control signal.
Each of the above optical amplifiers 102S and 102R is generally arranged as an optical fiber amplifier doped with rare-earth elements such as those of EDFA (Erbium Doped Fiber Amplifier). To this end, a Raman amplifier can be utilized together with the above optical amplifiers 102S and 102R.
In the WDM optical transmission system arranged as described above, the WDM signal (main signal light) is transmitted through the optical transmission paths 200 and 300 while relayed and amplified by the amplifiers 102S and 102R of the respective nodes 100A and 100B. Also, the monitoring control light (a signal having a lower transmission rate than that of the main signal light) is transmitted as the OSC signal or the like. The stations 100A and 100B, which can be located on the downstream side relative to the other, can detect an abnormal state brought about in the transmission path or the status of the upstream side by monitoring the monitoring control signal. Also, the stations 100A and 100B can notify the above-described information (monitoring information) to the station located on the upstream side thereof by using the opposing optical transmission path 300 or 200. For example, the node 100B can add information of the node 100B to information sent from a station C located on the downstream side and sends the resultant information to the station 100A on the upstream side.
In the case of the optical transmission nodes 100A and 100B where they are distant from each other or they perform the WDM transmission, the transmission is inevitably performed at a large transmission power. For this reason, in order for taking the security of a maintenance engineer or the like of the apparatus into consideration, a light beam emission at a high transmission power shall be prevented from being inadvertently brought about upon fiber pull-out, fiber cut or the like. To this end, as is disclosed in the following listed Patent Documents 1 to 3, a function is ordinarily provided so that the optical output is lowered or shutdown when the optical output is opened at the end of the fibers 200 or 300 to the space.
As for example shown in FIG. 11, if some accidents are brought about such that a fiber is cut at an output port of a station on the upstream side, an optical transmission path is cut, a fiber connection (connector) is inadvertently disconnected (step A1), and the receiving module 104R in the station 100B on the downstream side becomes unable to receive the monitoring control signal (step A2), this information is transmitted to the transmitting module 105S of the station on the opposite side (step A3). Then, the transmitting module 105S notifies the station 100A on the upstream side (receiving module 104R) of information that the station 100B on the downstream side 100B detects monitoring control signal down through the transmission path 300 on the opposing side by means of the monitoring control signal (step A4).
In this way, the receiving module 104R of the station 100A on the upstream side recognizes that the station 100B on the downstream side detects the monitoring control signal down (step A5). Then, the station 100A on the upstream side takes some countermeasure such as halting control on the optical amplifier 102S for the opposite side and the optical output to the optical transmission path 200 is shutdown (step A6). The control performed in the above-manner is known as an APSD control.
The APSD control may be performed not only for detection on the monitoring control signal down but also for detection on the main signal down. For example, if main signal light down is brought about in the input path to the optical amplifier 102R, information indicative of this incident is supplied to the station 100A on the upstream side by means of the monitoring control signal, and the optical output to the optical transmission path 200 of the station 100A on the upstream side is shutdown (see reference numerals 400 and 500 in FIG. 11). The above shutdown control is performed in response to a trigger which is made by detection of fresnel reflection at a portion where the connector is disconnected (e.g., see paragraph number 0009 of the following Patent Document 3). The following Patent Document 4 discloses a technology concerning reflected light detection, for example.
Further, if the arrangement of the two stations is one in which bidirectional transmission is performed between the two stations, in order to avoid control disabled status when double failure is brought about (i.e., the light down is brought about at transmission paths in the both directions), ordinary optical output decreasing control on the self node is performed in the upstream-side station but also in the downstream-side station in which the monitoring control signal down is detected (e.g., see paragraph number 0037 of the following Patent Document 2).
As for example shown in FIG. 12, it is assumed that some accidents such as connector disconnection and fiber cut are brought about in the optical transmission path 300 which is one of the transmission paths (step B1). Under this state, if some accidents such as connector disconnection and fiber cut are also brought about in the optical transmission path 200 which is the other of the transmission paths, there is no means for notifying the station 100A of the monitoring control signal down information, which might be detected by the station 100B if connection of the optical transmission path 200 is maintained. Therefore, it becomes impossible to effect the optical output control for decreasing the optical output from the station 100A.
For this reason, if the station 100A (the receiving module 104R) detects that it becomes incapable to receive the monitoring control signal from the station 100B (step B2), the station 100A takes some countermeasures such as halting control on the optical amplifier 102S for the opposite side so that the optical output from the station 100A itself is decreased (step B3).
The above-described APSD control is necessary for securing safety in the maintenance environment. However, if an output fiber is opened to the space upon set-up confirmation of the apparatus or a trouble management for the apparatus, the station on the downstream side will detect the monitoring control signal down. As a consequence, the optical output will be automatically decreased owing to the APSD control, with the result that it becomes unable to perform power measurement or the like.
As for example schematically shown in FIG. 13, when the station 100A is connected at an optical output terminal thereof with an optical power meter 600 through an optical fiber cord 201 for measuring the optical output power of the station 100A, the optical transmission fiber 200 is brought into an open state. Thus, the station 100B detects the monitoring control signal down, and information indicative of the monitoring control signal down is transmitted to the receiving module 104R by way of the optical transmission path 300. Therefore, the station 100A halts the optical output thereof owing to the APSD control and with the result that it becomes unable to perform power measurement.
For this reason, in general, the stations 100A and 100B are arranged to respond to an APSD inhibition (invalid) setting made from a maintenance terminal 700 or the like so that the APSD control can be forcibly brought into the inhibition status. In this case, if restoration is made from the inhibition status (APSD activating setting), the fundamental process is to make a restoration setting from the maintenance terminal 700. However, in order to avoid accidents such that the maintenance engineer forgets restoration setting or that the inhibition status setting is permanently left activated after the in-service mode (after starting the ordinary management mode), a timer is cooperatively operated for counting a time period of several minutes (i.e., the inhibition status setting validity time is set) for ensuring the automatic restoration from the inhibition status setting. The aforesaid inhibition status setting validity time is often set to several minutes on the basis of default, but this setting may be changed to several ten minutes or the like in accordance with a command inputted through the maintenance terminal 700.
Patent Document 1: Japanese Patent Application Laid-open No. 2001-77731
Patent Document 2: Japanese Patent Application Laid-open No. 2002-252595
Patent Document 3: Japanese Patent Application Laid-open No. 2003-8518
Patent Document 4: Japanese Patent Application Laid-open No. 2003-298527
However, even if the APSD control is arranged to restore automatically within a period of time of several minutes counted by the time as described above, this APSD control arrangement cannot always handle all possible accidental cases. That is, if the timer setting (inhibition status setting validity time) is set to a long period of time or other conditions are taken place and an engineer under the measurement duty at first leaves from the measurement work place or the engineer under the measurement duty is replaced with other person, the replacing person can unintentionally pull out the optical fiber cord 201 for the measurement in innocence of that the APSD control is canceled. In such a case, there is a fear that an optical output with a high power can be outputted.
The present invention is made in view of the above aspect. Therefore, it is an object of the present invention to propose a method in which even if the setting time for restoration is still not counted by the restoration timer, when the transmission path fiber is restored to its original state (reconnected) after the optical output power measurement or the like, the APSD canceling state is automatically restored to the ordinary state so that the optical output can be prevented from being outputted inadvertently from the fiber at a high power.